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Lecture 7 thermal insulation

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Lecture 7 thermal insulation

  1. 1. THERMAL INSULATION June 5th, 2017 Lecture 7 Ephrem M.
  2. 2. The polar bear has a thick body fur to insulate himself from the chilly environment around. Eskimo lends this fur to keep his body warm, which will be in stark contrast with the surrounding. A casual observation around clearly shows why women are less panicky when it comes to handling hot things; it is because of their insulating subcutaneous fat. The equivalent analogy on this biological systems is the insulation of buildings from their natural setting. THERMAL INSULATION
  3. 3. The sheathing often placed around steam and hot-water pipes, for instance, reduces heat loss to the surroundings, and insulation placed in the walls of a refrigerator reduces heat flow into the unit and permits it to stay cold. THERMAL INSULATION Apart from geometrically responding to a harsh climate by manipulating the form, insulation of buildings from the environment is a wise insight in energy conscious world of the day. Thermal insulating materials are used to reduce the flow of heat between hot and cold regions.
  4. 4. What is Thermal Insulation? Where there is difference in temperature of inside of a building and outside atmosphere, heat transfer takes place from areas of higher temperature to lower temperature. When the external temperature is low, the internal temperature should be high so as to produce thermal comfort. Thermal insulation indicates the construction or provisions by way of which transmission of heat from or in the room is retarded. Thermal insulation can be attained during the design process from providing thermal insulating materials on the “shell” of the building.
  5. 5. General methods of Thermal Insulation 1. heat insulation by orientation 2. Heat insulation by shading 3. Heat insulation by proper height of ceiling
  6. 6. General methods of Thermal Insulation 1. heat insulation by orientation 2. Heat insulation by shading 3. Heat insulation by proper height of ceiling 1. internal building surfaces which can receive direct incident solar radiation from winter to summer 2. Optimum inclination of solar cells for global radiation throughout the year the orientation of the building with respect to the sun has a very important bearing on its thermal behaviour. For optimum orientation, there are usually conflicting requirements. Minimum transfer of solar heat desired during the day in summer, while maximum heating of rooms by solar heat is required during winter.
  7. 7. 1. heat insulation by orientation 2. Heat insulation by shading 3. Heat insulation by proper height of ceiling Shading of roof brings down the surface temperature, but it is very difficult to achieve this effect in practice, especially when the altitude angle of the sun is high during the peak heat gains in the afternoon, between 1100h and 1500h. Raising the parapet walls can help only when the altitude angle of the sun is low. General methods of Thermal Insulation
  8. 8. 1. heat insulation by orientation 2. Heat insulation by shading 3. Heat insulation by proper height of ceiling While the surface temperature of the ceiling does not vary with its height, the intensity of the long wave radiation, emitted by the ceiling decreases as it travels downwards. The effect of vertical gradient of radiation intensity is not significant beyond 1 to 3m. Hence it should be adequate to provide ceiling at a height of about 1 to 1.3m above the occupant. General methods of Thermal Insulation
  9. 9. Advantages of Thermal Insulation ✓It provides comfort as it keeps room: ▪ cool in summer and ▪ hot in winter. ✓It saves fuel by minimizing heat transfer, ✓prevents condensation or moisture deposition on interior walls and ceilings and ✓it prevents freezing of water taps in extreme winter, and heat loss in case of hot water system.
  10. 10. Thermal Insulation functions 1. reduce thermal conduction in the material in which heat is transferred by electrons; 2. reduce thermal convection current, which can be set up in air- or liquid-filled spaces 3. reduce radiation heat transfer in which thermal energy is transported by electromagnetic waves.
  11. 11. Thermal Insulation functions Conduction and convection are suppressed in a vacuum, in which radiation is the only method of transferring heat. If the surfaces are made highly reflective, radiation can also be reduced. Thus, thin aluminium foil can be used in building walls, and reflecting metal on roofs minimizes the heating effect of the Sun.
  12. 12. Thermal Insulation functions Air offers about 15,000 times as much resistance to heat flow as a good thermal conductor such as silver does, and about 30 times as much as glass. Typical insulating materials, therefore, are usually made of non- metallic materials and are filled with small air pockets. They include magnesium carbonate, cork, felt, cotton batting, glass wool, and diatomaceous earth.
  13. 13. Thermal Insulation functions
  14. 14. Thermal Insulation functions In building materials, air pockets provide additional insulation in hollow glass bricks, double-glazed windows (consisting of two or three sealed glass panes with a thin air space between them), and partially hollow concrete tile. Insulating properties become poorer if the air space becomes large enough to allow thermal convection, or if moisture seeps in and acts as a conductor.
  15. 15. Thermal Insulation functions Home-heating and air-conditioning costs can be reduced by proper building insulation. In cold climates about 8cm (about 3in) of wall insulation and about 15 to 23cm (about 6 to 9in) of ceiling insulation are recommended. Super insulation has recently been developed, primarily for use in space, where protection is needed against external temperatures near absolute zero. Super insulation fabric consists of multiple sheets of aluminized Mylar, each about 0.005cm (0.002in) thick, separated by thin spacers, with about 20 to 40 layers per cm (50 to 100 layers per in).
  16. 16. Thermal Insulation functions
  17. 17. Thermal Insulation functions 1. Low temperatures: insulation for vessels containing cryogenic materials, such as liquefied natural gas 2. Ambient temperature: insulation for building structures 3. Medium temperature: insulation for tanks, pipes, and equipment in industrial process heat applications 4. High temperatures: refectory or other specialized insulation materials used in foundry work, nuclear power facilities, the aerospace industry Thermal insulation controls heat flow under temperatures ranging from absolute zero to 3000oF (1600oC). This broad range can be subdivided into four general temperature regimes that classify applications for various types of insulation;
  18. 18. Three points to consider in using Thermal Insulating Materials  Where is thermal insulation used?  How does insulation work?  What is the R-value (Thermal resistivity)?
  19. 19. Use of thermal insulation If the air outside is cold, you may want to protect your skin by wearing clothes that keep the cold out and the body warmth in. If your house has cool air inside during the summer, you may want to prevent the temperature from becoming the same as the hot air outside by having the house well insulated. You may want to prevent hot drink from becoming room temperature by putting it in a thermos bottle. Three points to consider in using Thermal Insulating Materials
  20. 20. How Does Insulation Work? Three points to consider in using Thermal Insulating Materials Insulation is a barrier that minimizes the transfer of heat energy from one material to another by reducing the conduction, convection and/or radiation effects. Most insulation is used to prevent the conduction of heat. Less dense materials are better insulators. Thus, gases insulate better than liquids.
  21. 21. How Does Insulation Work? Conduction occurs when materials--especially solids--are in direct contact with each other. To slow down the transfer of heat by conduction from one solid to another, fiberglass and vacuum are mostly used. Insulation from conduction Three points to consider in using Thermal Insulating Materials
  22. 22. How Does Insulation Work? Radiation is transfer of heat from warm objects by radiating infrared electromagnetic waves which can be prevented by a shiny material to prevent radiation heat transfer. Insulation from Radiation Three points to consider in using Thermal Insulating Materials
  23. 23. R-Value The R-value of a material is its resistance to heat flow and is an indication of its ability to insulate. • It is used as a standard way of telling how good a material will insulate. • The higher the R-value, the better the insulation. Its units of measurement: ▪ (square feet x hour x degree F)/BTU in the English system ▪ (square meters x degrees C)/watts in the metric system Three points to consider in using Thermal Insulating Materials
  24. 24. R-Value Three points to consider in using Thermal Insulating Materials Material R-Value Material R-Value Hardwood siding (1 in. Thick) 0.91 Cellulose fibre (1 in. Thick) 3.7 Wood Shingles (lapped) 0.87 Flat glass (0.125 in. Thick) 0.89 Brick (4 in. Thick) 4 Insulating glass (0.25 in space) 1.54 Concrete block (filled cores) 1.93 Air space (3.5 in. thick) 1.01 Fiberglass batting (3.5 in. Thick) 10.9 Free tagnant air layer 0.17 Fiberglass batting (6 in. Thick) 18.8 Drywall (0.5 in. Thick) 0.45 Fiberglass board (1 in. Thick) 4.35 Sheathing (0.5 in. Thick) 1.32
  25. 25. Thermal Insulating Materials 1. slab or block insulation—known as blocks or boards, 2.5cm thick and 60cm x 120cm or more in area. These may be made of cork board, mineral wool, vermiculite, cellular glass, cellular rubber, saw dust, asbestos cement, etc. these are fixed to walls, slabs or roofs. slab or block insulation
  26. 26. Thermal Insulating Materials slab or block insulation
  27. 27. Thermal Insulating Materials 2. Blanket insulation—these are flexible fibrous rolls made from mineral wool, processed wood fibres, cotton, animal hair, etc. available in thickness of 12 to 80mm. These are directly spread on the wall or ceiling surfaces.
  28. 28. Thermal Insulating Materials 3. Loose fill insulation—may consist of fibrous materials like rock wool, slag wool, cellulose or wood fiber wool, etc. filled loosely in the studding space.
  29. 29. Thermal Insulating Materials 4. Batt insulation—similar to blanket insulations except that these are small in size but of greater thickness. These are also spread on surface of walls and ceiling. Batt insulation
  30. 30. Thermal Insulating Materials 5. Insulating board—used for interior lining of walls and also for partition walls, available in different sizes and thickness 6. Light weight aggregate—heat resistance of concrete can be greatly increased by using blast furnace slag, burnt clay, vermiculite
  31. 31. Thermal Insulating Materials 7. Reflective sheet—have high reflectivity and low emissivity, thus offering high heat resistance. It consist of gypsum boards, steel sheet, aluminium foils, etc. Unlike others, reflective sheets rely on their surface characteristics, thickness of air space & temperature differences for their insulating value.
  32. 32. Thermal Insulating Materials 8. Air barrier—effectively retard or stop the flow of warm, moisture- laden air from inside a building outward through walls, ceiling, and floors to the colder, dryer outside atmosphere.
  33. 33. Application of Insulation materials 1. Flexible insulation – provided in open spaces in the wall, ceiling, or roof construction where it will not be subjected to loads, as it is easily compressible. Quilts or blankets are put between furring strips on masonry wall. Blanket insulation divides the space into two or more air spaces. For greater efficiency, the insulation is fastened at the sides and ends so that there will be no air leakage between spaces. Batts are placed between studs
  34. 34. 2. Fill insulation - installed by pouring the loose granulated material into open spaces between the studs or between ceiling joists. The material may be blown into place through large flexible tubes employing low air pressure. Care is taken to insure the complete filling of space between stud. Application of Insulation materials
  35. 35. 3. Reflective insulation - fixed between studs, joists or rafter. It is held more tightly by using nailing strips. This type of insulation divides the space into two or more air spaces. It is essential that the insulation is fastened at the sides, top, bottom and all laps to restrict the air circulation between the two spaces. Application of Insulation materials
  36. 36. Choice of Insulating materials The choice of insulating material depends upon: ➢the cost of material, ➢area to be covered, ➢standard of insulation required and ➢coat of heating and cooling.
  37. 37. Properties of Insulating materials ✓high thermal resistance, ✓reasonably fireproof, ✓insect proof, ✓durable, ✓non-absorbent of moisture, ✓cheaper and readily available. The insulating material Should have properties of: It should be noted that low density insulating material give better insulations than high density materials. Also the presence of air spaces in the insulating material increases thermal insulation while presence of moisture decreases thermal insulation.

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